Cord fabric is used as tire reinforcement material. Since the chemical structures of synthetic fibers used in cord fabric production and rubber are considerably different from each other, the said materials are incompatible with each other in terms of their chemical and physical structures. Synthetic fibers have high strength and low elongation whereas rubbers are polymeric materials which have high elongation and low strength. The polar groups (amide, hydroxyl and carbonyl groups) present in the structure of the synthetic fibers are incompatible with the non-polar structures of the rubber. This incompatibility present in the state of the art is eliminated with water-based Resorcinol-Formaldehyde-Latex (RFL) adhesive solutions which form a phase between the cord and the rubber and enable the rubber and the fiber to be attached to each other.
Main function of RFL is to serve as an adhesive bonding two incompatible structures by forming a phase between fiber and rubber. RF functional group in RFL is attached to the polar groups of fiber while Latex (L) group is attached to the fiber by vulcanization, and thus the rubber-fiber composite structure is formed. The vehicle tire application is amongst the most significant product examples wherein the said composite structure is used. The water based RFL adhesives are applied on cord fabric during “dipping” process which is the final step of cord fabric production. The strength of the bonds formed between the rubber and the cord is examined with adhesion tests. Adhesion is a very important parameter in high tenacity cord reinforced rubber products. This is because cord-rubber adhesion is a factor which directly effects the tire life and performance.
RFL adhesive formulation has been used as an adhesive material in all synthetic fiber reinforced materials for over half a century because of its stable structural features and low cost. However, both resorcinol and formaldehyde are the chemicals which possess high risk for human and environmental health, and therefore their use is limited. Regarding this subject, significant feedbacks have come from international organizations, manufacturers and end users. It is known that the resorcinol causes itching and rash when it contacts the skin, irritates the eye and shows toxic properties in liver and cardiovascular systems.
Formaldehyde is riskier than resorcinol for human health and safety. In 2004, formaldehyde was classified as group 2A chemical (probably carcinogenic to humans) by a group of scientists in International Agency for Research on Cancer (IARC) of World Health Organization, and later as group 1 (carcinogenic for humans). In 2009, formaldehyde was included in the list of chemicals causing leukemia by IARC. In line with this, formaldehyde was claimed to be a gene mutagen. Even low level of formaldehyde (1 ppm) can cause eye, nose and throat irritation.
Although formaldehyde-based resins are advantageous in terms of cost, both producers and consumers search for alternatives because of the reasons stated above. Therefore, especially in recent years, researchers have been working on the development of resorcinol and formaldehyde-free cord fabric dip solutions. The preparation of formaldehyde-free dip solutions have been reported in various studies.
United States Patent document no US20120041113, an application known in the state of the art, discloses preparing a composition comprising an epoxy, a blocked isocyanate, an epoxy curing agent and vinyl pyridine latex.
International Patent document no WO9600749, another application known in the state of the art, discloses the application of dipping solutions formulated with three functional-epoxy resins, styrene-butadiene-vinyl pyridine and styrene-butadiene-acrylonitrile-acrylic acid latex to polyethylene terephthalate (PET) cord fabric and its strength of adhesion with rubber.
United States Patent document no U.S. Pat. No. 5,118,545, another application known in the state of the art, discloses the synthesis of an aramide comprising multiple double bonds. It is stated that the synthesized resin is applied on the aramide-based cord fabric and that the double bonds in the aramide resin are vulcanized with the double bonds in rubber while the amide groups provide physical adhesion to the aramide fiber.
U.S. Pat. No. 4,472,463 discloses dipping non-adhesive activated PET fibers with two-step dipping process. The first dipping step comprises aromatic glycidyl ester epoxy and blocked isocyanate, while the second dipping step comprises two different latexes. The first latex is styrene-butadiene-vinyl pyridine copolymer, and the other one is acrylic acid-methyl methacrylate-styrene copolymer. It is stated that the H-adhesion values are higher than that of RFL dipped fibers.
United States Patent document no US20040249053, another application in the state of the art wherein an environmentally friendly dipping material is disclosed, discloses that the maleinized-polybutadiene is rendered water-soluble by reacting with polyethylene glycol. The PET cords modified with epoxy are first coated with this resin and then with styrene-butadiene-vinyl pyridine latex. The said resin exhibited lower adhesion strength relative to the fabrics treated with RFL.